The present invention relates to a method for relieving residual stresses of an inner surface of a weld by build-up welding of an outer surface of the butt welded joint between pipe sections.
When the pipe sections are butt welded, high tensile residual stresses are left in the weld and the heat affected zone. If the residual stresses are not relieved, stress corrosion cracking results so that in the cases of pipes and cylindrical or tubular vessels used in nuclear reactors, chemical plants or the like, there exists a fear that a serious accident will happen. Therefore, the residual stresses must be relieved. Generally the welded joints are subject to such heat treatments that the tensile residual stresses in the inner surface of the weld and the heat affected zone are relieved or converted into compressive residual stresses.
There are various conventional residual stress relief methods such as a high frequency heating process, a shot peening process and a (Linde) process for heating the outer surface of a pipe with flames. The high frequency heating process requires a large-sized equipment and a high frequency induction coil must be prepared for each pipe so that the high frequency heating process is expensive. Furthermore, the high frequency heating process requires many days. According to the shot peening process, the inner surface of a pipe must be subjected to the shot peening. Therefore, the shot peening process cannot be applied to a pipe in service. The Linde process cannot relieve the compressive residual stress in a stable manner.
In view of the above, the present invention provides a very simple yet very effective method for relieving residual stresses in a weld. According to the present invention, the outer surface of a weld is subjected to build-up welding under predetermined conditions.